Salmonella infections have been prevalent in poultry flocks for several decades. Recently, Salmonella enteritidis (S. enteritidis) infections have caused epidemics in the United States and the United Kingdom. Outbreaks of food poisoning in humans due to the consumption of grade A eggs have increased dramatically. Epidemiological and microbiological investigations have shown that the outbreaks have involved foods containing hen's eggs, and approximately 80% have been caused by S. enteritidis. See St. Louis, M. E. et al. 1988. The Emergence of Grade A Eggs as a Major Source of S. enteritidis Infections. J. Amer. Med. Assoc. 259:2103-2107 and Coyle, E. F. et al. 1988. S. enteritidis Phase Type 4 Infection: Association with Hen's Eggs. Lancet, Dec. 3, p. 1295-1296.
The poultry industry urgently needs to find means to control infections of poultry flocks and eggs. The gastric tract of poultry is a major reservoir of human pathogens. Feed is often the source of contamination, which leads to intestinal infection or colonization in the chicken. In turn, virulent pathogens may enter the blood and oviduct by passage through the cecal mucosa. Much effort has been concentrated on the elimination of pathogens from the environment, feeds and gastric tracts of poultry. Extensive research has been performed on providing competitive flora, referred to as the "Nurimi Concept." However, infections still persist.
In order to understand the present invention, it may be helpful to briefly review some features of bacteria and antibiotics. A bacterium is a unicellular microorganism which may exist either as a free-living organism or as a parasite. Bacteria, or a single bacterium, have a wide range of biochemical and pathogenic properties.
Bacteria may be classified according to cell wall structure into two basic groups: gram-negative bacteria and gram-positive bacteria. Gram-negative bacteria have more complex cell wall structures than gram-positive bacteria. In particular, the cell wall of gram-negative bacteria have an outer membrane, which is absent from gram-positive bacteria.
The outer membrane of a gram-negative bacterium includes inner and outer protein layers with phospholipid (a type of fat) disposed in a bilayer between the protein layers. The proteins may include matrix protein (also referred to as porin) which is transmembranal and may create pores in the membrane to allow diffusion of molecules through the outer layer. Lipoproteins may anchor the outer membrane to a peptidoglycan layer. The outer membrane, lipoprotein, peptidoglycan, and periplasmic space, which are all layers external to the inner membrane, may be referred to as the cell wall.
The outer membrane of the cell wall includes lipopolysaccharide molecules which extend from the cell wall. An outermost portion of a lipopolysaccharide molecule is referred to as an O antigen polysaccharide. O antigens are specific polysaccharide side chains and are often the major antigenic determinants in gram-negative bacteria.
The outer membrane may repel certain compounds. For instance, the lipopolysaccharide molecules are hydrophilic (water-loving). Hence, the lipopolysaccharide molecules may deter the approach of hydrophobic compounds. However, even hydrophilic compounds of a size greater than the pores created by the matrix protein may be excluded by the outer layer.
Salmonella is gram-negative group of bacteria with more than 2,000 serotypes. In biological nomenclature, Salmonella is a genus which contains only one species. The one species comprises seven subspecies. Each subspecies is divided into serovars. Serovars are further divided into serotypes. S. enteritidis has been identified as a serovar.
An antibiotic is a naturally produced substance which is effective in inhibiting the growth of or destroying microorganisms. Antibiotics may be produced by fungi, bacteria, or other organisms.
An antibiotic such as polymyxin may damage or kill a bacterium such as by altering its membrane permeability, impeding respiration or impairing certain transport phenomena such as electron transport of the microorganism. For example, an antibiotic may bind to the outer membrane of a bacterium and open holes in the membrane through which components of the cell may leak out, thereby killing the bacterium. In other words, the permeability of the outer and inner membranes for small-molecular weight compounds may be increased in the presence of antibiotics and the efflux of cytoplasmic components may occur.